How to invoke custom node operator
The solution I've found was to to override the context.node call. The way I did it was by creating a dictionary with 'node' as the key and the node as the item, e.g.
override = {'node': bpy.data.node_groups[node_group_name].nodes['node_name']}
The operator can then be run with bpy.ops.node.ligexport(override, 'INVOKE_DEFAULT']).